Homeostatic signaling systems are crucial forms of biological regulation that permit flexible yet stable information transfer in the nervous system. These fundamental mechanisms operate to maintain such properties as synaptic strength and glutamate levels within stable physiological ranges. Although intensive research has been focused on understanding how excitatory synapses are homeostatically modulated to stabilize synaptic strength, far less is known about how these synapses adjust to control glutamate release itself. Excess glutamate release can lead to a variety of diseases and dysfunctions in the nervous system, contributing to seizures, excitotoxity, and neurodegeneration. Here, we propose to characterize a glutamate homeostat that controls presynaptic function using the Drosophila neuromuscular junction as a unique and powerful model system. At this glutamatergic synapse, excess presynaptic glutamate secretion induces a homeostatic inhibition of neurotransmitter release, an adaptation referred to as presynaptic homeostatic depression (PHD). This process parallels a similar phenomenon observed in a variety of other organisms, including mammalian central synapses. We hypothesize that excess glutamate is sensed by a presynaptic glutamate receptor and activates an autocrine signaling system to homeostatically depress synaptic vesicle release. To test this model, we will use a systematic electrophysiology screen to test glutamate receptors in Drosophila for roles in PHD. Next, we will leverage a combination of cell biology, heterologous expression, pharmacology, and innovative functional imaging techniques to determine the mechanisms through which excess glutamate signals a precise reduction in presynaptic vesicle release. Finally, we will assess how synapses, neurons, and glia adapt to chronic glutamate imbalance using several approaches, including a cell-specific translational profiling technology we have developed as well as a new generation of glutamate indicators. Together, these experiments will advance our understanding of the mechanisms that endow synapses with the ability homeostatically tune glutamate release, and will identify maladaptive responses to glutamate imbalance in the nervous system. Ultimately, this knowledge will inform therapeutic strategies towards counteracting diseases associated with glutamate imbalance, including epilepsy, fragile X syndrome and neurodegeneration.